1. Field of the Invention
This invention relates to the regulation of the velocity of electric motors and especially to a control system for operating a DC electric motor at a constant velocity under variable loading conditions.
2. Discussion of Prior Art
Many applications for electric motors require that the motor be operated at a constant velocity notwithstanding changes in the loading of the motor. One particular application involves recording of visible information on a record medium such as electrically-sensitive paper. In such recording devices an electrode in contact with the electrically-sensitive paper is activated to produce marks on the paper while the electrode is moved at a constant velocity over the surface of the paper. During the recording operation the relative motion between the recording element and the paper must be maintained constant although the recording operation may impose variable loading conditions on the drive motors. To avoid distortion of the visible information the constant relative velocity between the record medium and the recording elements must be maintained over all or an extended portion of the length of the recording medium notwithstanding the variability of the loading conditions.
Various systems for achieving this constant velocity as well as for other constant velocity applications have been devised. It is well known is such systems to generate a feedback signal in conjunction with the operating of the motor which represents the actual velocity of the motor. The velocity indicating feedback signal is compared with a reference signal of basically the same kind which represents the desired velocity of the motor. The comparison is made between the feedback signal and the reference signal to produce control pulses which operate a power source which applies energy to drive the motor. If the feedback signals vary from a predetermined standard as represented by the reference signal, correction is made to vary the power source to increase or decrease in accordance with the change in velocity.
Examples of motor velocity control systems which use a reference signal are shown in U.S. Pat. No. 3,753,067 issued Aug. 14, l973 and U.S. Pat. No. 3,241,023 issued Mar. 15, l966. In both patents the reference signal is provided by an external signal generator which operates at very precisely defined parameters. Such reference signal sources represent a substantial cost item in the total cost of the velocity control system. Likewise, the precision of the reference signal source tends to vary over extended time.